Process for wet grinding solids to extreme fineness



3,075,710 FINENE'SS Jan. 29, 1963 l. L. FELD ETAL PROCESS FOR WET GRINDING so Filed July 18, 1960 LIDS TO EXTREME 2 Sheets-Sheet. 1

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PROCESS FOR WET GRINDING SOLIDS T0 EXTREME FINENESS Filed July 18, 1960 2 Sheets-Sheet 2 INVENTORS 1 IGNA 72 Louis FELD 54 L L 4170 HERSGHEL GLEMMO/VS MW MM United States Patent 3,075,710 PRGCESS FGR WET GRINDING SQLIDS T0 EXTREME FEhIENESS Ignatz L. Feld, Tuscaloosa, Ala, and Ballard H. Clemnrons, Salt Lake City, Utah, assignors to the United States of America as represented by the Secretary of the Interior Filed Italy 18, 1960, Ser. No. 43,696 15 Claims. (Cl. 241-16) (Granted under Title 35, US. Code (1952), sec. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for govern-mental purposes without the payment of royalties thereon or therefor.

This invention relates to a new and improved wet process for reducing the particle size of solids to extreme fineness and more particularly to the wet grinding of kaolinitic clays.

In recent years kaolinitic clays have been the major source of paper coating clay which is used in the manufacture of coated papers. Of the clays mined for the production of paper coating clay, only one-half to three- =fourths of the clay is suitable for use as paper coating clay. Kaolintic clays suitable for paper coating may contain as little as 70 percent by Weight minus-Z-micron e.s.d. (equivalent spherical diameter) material or more than 90 percent by weight minus-Z-micron e.s.d. material.

In most instances the portion of clay rejected as unsuitable is essentially all kaolinite but contains an excessive quantity of plus-Z-micron e.s.d. material, which may be as much as 86 percent by weight of material with this particle size. Although this relatively coarse clay is sometimes sold as a filler clay, its monetary value would be considerably increased if it could be ground to a finer particle size and marketed as a paper coating clay. It has been estimated that the reserves of paper coating clays in the Southeastern United States could be increased as much as 25 percent, by the development and application of a satisfactory method for reducing the particle size of coarse Georgia kaolins into the minus-Z-microns e.s.d. range. This invention has as an object, the grinding of coarse kaolintic clays for the production of finer and more desirable clays acceptable as a paper coating clay, although grinding to a kaolinitic clay product containing quantities of up to about 37 microns e.s.d. material is also contemplated. It is recognized that other processes for reducing the particle size of solids to extreme fineness are commonly in use. Some typical wet processes include the use of equipment such as pebble mills, ball mills, tube mills, colloid mills, roller mills, etc. Each of these processes has some feature disadvantages to its use for the fine grinding of solids. These disadvantages include low production rate, high power consumption, large investment cost, poor grinding eificiency, contamination of product, etc.

This invention has as an object, the development of an improved process for wet grinding of solids to produce increased quantities of extremely fine sized material in the ground product. It is a further object of this invention to provide a wet grinding process using simply con structed, low cost equipment. It is a still further object of this process to provide a wet grinding process with large feed capacity and relatively low power consumption. It is a still further object to provide a wet grinding process which produces a high rate of particle size reduction and very low contamination of the ground product. A still further object of this invention is to provide a wet grinding process for grinding clays for the production of a finer clay more suitable for use as a ceramic, filler or other "ice type clay. A still further object of this invention'is to provide a process for wet grinding coarse kaolinitic clay for the production of finer and desirable ground kaolinitic clay acceptable as a paper coating clay.

Specifically, this invention provides a process for grinding relatively coarse kaolinitic clay to extreme fine size. The grinding is accomplished in an agitator which subjects the clay to intense agitation in a slurry composed of water, clay and minus-14-plus-28-mesh size granular sand.

Other objects, purposes, and desirable features of the invention will appear hereinafter and will be apparent from the subsequent description.

The invention will be best understood by examination of the following detailed description used in connection with the accompanying drawing forming part of this specification, with the understanding, however, that the invention is not necessarily confined to any strict conformity with the drawing but may be changed or modified so long as such changes or modifications make no material departure from the salient features of the invention as eX-- pressed in the appended claims.

FIGURE 1 is a side elevation, partly in section, of a simple and etficient apparatus for batch operation.

FIGURE 2 is a plan view of the apparatus shown in FIGURE 1.

FIGURE 3 is a side elevation, partly in section, of a simple arrangement of apparatus for the continuous efiicient grinding of solid materials.

Referring to FIGURE 1, the apparatus is composed of a cylindrical vessel 2 with an open top. The vessel 2 is lined with a rubber or equivalent abrasion resistant material 3 at the sides and at the bottom. The shaft 1 is rotated by some suitable source of power such as an electric motor. Attached to the lower end of shaft 1 is a rotor 9 which is a cage-like structure composed of a solid disc at the top 6 and a lower disc 8 at the bottom, both discs 6 and 3 have the same diameter. However, the lower disc 8 has a circular opening in the center. These two discs, 6 and 8, are rigidly held together by a multiplicity of vertical bars 7 which are fixed in notches at the periphery ofthe upper disc 6 and the lower disc 8. This cage-like rotor 9 is turned by shaft 1 inside but without touching a stator 10 composed of an upper ring 5 and a lower ring 11 held together by a plurality of vertical bars 4. A discharge opening with a valve 12 is located in the center at the bottom of the grinder vessel 2.

Referring to FIGURE 3, the apparatus is composed of a grinding unit similar to that shown in FIGURES 1 and 2; however, the cylindrical container vessel has been extended upward to provide settling space above the grinding zone where the rotor 9 and stator 10 are located. The purpose of this space is to promote settling of the relatively coarse grinding media so that it will remain within the grinder and not overflow at 17 with the ground solids. A grid 25 to facilitate settling is located somewhat above the stator It). The slurry of water and ground solids overflows from the grinder at 17. This slurry is conducted to a centrifuge 18 in which the finished fine solids are separated into a slurry of water and solids that is discharged at 20. A slurry of water and the coarse unfinished solids is discharged at 19. The coarse solids product is conducted to a suitable container 16 where fresh unground solids 21 are added as feed with the feeder '22; make-up water is also added here from source 23. This mixture of coarse solids from the centrifuge 18, plus fresh feed 21, plus make-up Water 23 are conducted to the bottom of the grinder where they enter the grinding zone through entrance 15. Defiocculating reagent or reagents may be introduced into the process either as a solid or as a solution by adding to the mixture of material at 16. A circumferential jacket 13 surrounds the sides of the vessel 2 and is equipped with openings 24 and 14 which are used as inlet and outlet for introducing a temperature controlling fluid which may be used to control the temperature of the charge during processing.

In batch operation the equipment as shown in FIG- URE 1 is charged with a suitable mixture of grinding media, solids, water and dispersant, valve 12 is closed during this part of the operation. The rotor 9 is turned at a sufficient rate to attain a peripheral speed of about 1400 feet per minute. After the desired period of grinding has passed, the rotor is stopped and the valve 12 is openedto remove the grinding media and pulp. In practica-lly every instance the grinding media is much coarser than the solids being ground and hence is readily separated from the ground solids by screening orby hydraulic classification.

Continuous closed circuit grinding is accomplished in the equipment shown in FIGURE 3 by operating it with valves 12 and 26 closed and using the following procedure. The initial load, consisting of water, dispersant, grinding media, and solids to be ground are put into the vessel, the motor which turns rotor 9 at a peripheral speed of 1400 feet per minute is started after the addition of the water and dispersant. As the quantity of material added becomes sufiicient to overflow at 17 the centrifuge discharge 19', fresh feed 21, and make-up water 23 are added to 16 and drawn into the grinder through as valve 12 is gradually opened. During normal grinding operation a sufliciently low pressure is formed at the bottom of the grinder so that these materials 21, 19, and 23 may be drawn into the grinder. By means of this arrangement the solids that are ground suificiently fine are withdrawn from the process at 20 and the oversize material 19 requiring additional grinding is added to the feed 21 with make-up water 23 and returned to the grinding zone. Thus, closed circuit grinding is achieved and the grinding process is operating at high efiiciency, the power consumed per unit of ground material should be at a minimum, the grinding capacity of the unit should be at a maximum and the wear on the equipment should be a minimum per unit of solids ground.

A temperature controlling fluid, such as water, may be circulated through jacket 13 using openings 24 and 14 as inlet and outlet respectively. Thus the temperature at which the process takes place may be readily controlled.

Should any of the relatively coarse grinding media overflow with the ground slurry it will be separated in the centrifuge 1 8 into the coarse slurry product 19 and returned to the grinder through 1-6 and 15.

Upon completing the operation of the equipment, the charge of grinding media, solid-s and water may be drained and flushed out through valve 26.

The grinding media used in the process should be a granular material, harder than the solid material to be ground, spheroid in shape and have sufficient toughness to resist being broken down into smaller particles during the grinding process. If possible the media should be of a composition which would not act as an undesirable contaminant to the solid material being ground should any portion of the grinding media be reduced to a particle size fine enough to prevent its being removed from the ground product. Ottawa quartz sand has been used very satisfactorily, but this does not exclude the use of other quartz sands. Ottawa quartz sand is composed of naturally rounded particles, but angular shaped sands should be satisfactory for use after an initial period of grinding because the particles would approach a spheroid shape with continued use, as the angular portion would be abraded away at a greater rate than the other parts of the particle. Other suitable granular materials beside quartz sands may be used as grinding media; an incomplete list of these might include natural mineral sands of garnet, zircon, corundurn, and staurolite; synthetic granular materials might include fused A1 0 and mullite pellets.

We have found that grinding media in the particle size range of minus-ltmesh (Tyler standard screen scale with square openings measuring 0.046 inch on a side) plus-28- mesh (Tyler standard screen scale with square openings measuring 0.023 inch on a side) is satisfactory for grinding kaolinitic clays containing no more than 20 percent by weight minus-ZO-micron e.s.d. to a size range containing more than 90 percent by weight of material minus-2- micron e.s.d. We have also found that as the particle :size of the grinding media is reduced there is a tendency for more grinding to take place in the sizes even finer than 2 micron e.s.d., e.g., in the one micron e.s.d. range. Thus the size of the grinding media is selected on the basis of the ultimate fineness of grind and also the type of size distribution that is desired in the final product. We have found that efiective operation of the process is obtained when the particle size of the grinding media is from 705 to 75 times the diameter of the upper limiting particle size desired in the final ground product. For example, to grind a kaolini-tic clay so that the limiting upper size range of the major portion of the clay would be at 2 microns e.s.d., we would use a granular grinding media with particles of maximum diameter of 1410 microns (705 2 microns=l4l0 microns) or about 'l4-mesh (Tyler standard screen scale) and with particles of minimum diameter of 150 microns (75x2 microns=150 microns) or about IOO-mesh (Tyler standard screen scale).

The use of deflocculants with the grinding slurry is not mandatory but increases the efliciency of the grind. Good particle size reduction was obtained by the grinding process when a deflocculant such as tetrasodium pyrophosphate was used at the rate of 0.33 pound for every 100 pounds of clay. Variations in the quantity of dispersant used did not critically change the efficiency of grinding, but dispersion of the solids to be ground was necessary for most efiicient grinding. Various defiocculating reagents were found to be satisfactory in the grinding process. A partial list of these included sodium hexametaphosphatc, sodium triphosphate, sodium tripoly phosphate, sodium silicate and sodium carbonate.

We have found that efficient grinding takes place when the grinding charge is composed of 18 percent by weight of water, 19 percent by weight of clay and 63 percent by weight of quartz sand grinding media; this represents a grinding charge composition wherein the grinding media comprises 77 percent by weight of the total solids in the charge. A deflocculating reagent, tetrasodium pyrophosphate (0.33 gram per 100 grams of clay) was present in the slurry. It is not intended that the process shall be limited to this grinding charge combination of water, clay, grinding media, and deflocculating reagent as other combinations of water, clay, grinding media, and deflocculating reagent have been used satisfactorily in the grinding process. We have found that as a general rule an increased rate of particle size reduction is achieved when the ratio of grinding media to the material to be ground is increased; however, it is evident that this also decreases the capacity of the grinder. We have found that effective operation of the grinding process is obtained with grinding charges wherein the grinding media comprises from 48 to percent by volume of the total solids present in the grinding charge.

The following examples are given to illustrate more clearly the preferred modes of operation and the advantageous results obtained; however, it is not intended that the invention shall be limited by these examples of practice.

Example 1 Grinding equipment such as shown in FIGURES l and 2, with a container having an internal diameter of about 5 inches, was used to grind a sample of Georgia kaolinitic clay on a batchwise basis. The grinding charge had the following composition:

The size of the Ottawa quartz sand was minus-14- mesh/plus-ZB-mesh (Tyler standard screen scale). About 1.15 grams of tetrasodium pyrophosphate was added to the grinding charge to deflocculate the clay. The sand grinding media occupied 76 percent of the total volume of clay and sand in the charge.

After the valve at the bottom of the container (FIG- URE l, valve 12) was closed, the water and deflocculating reagent were first placed in the mill. Then the clay was added and the grinder was run for five minutes to thoroughly wet the clay. The quartz sand grinding media was then added to the mill and the rotor 9 was turned by hand until the grinding media was wet by the clay and Water slurry. The grinder was then turned on and the rotor turned at a rate suflicient to give a peripheral speed of about 1400 feet per minute. After a 30-minute grinding period the rotor was stopped; valve 12 was opened and the grinding charge flushed out of the mill onto a vibrating screen. The clay and water were drained away from the sand media on the screen, and the sand was sprayed several times with fresh water to remove essentially all of the clay from the sand media.

The particle size distribution was determined on a representative sample of the unground clay and also on a representative sample of the clay taken from the ground clay slurry. The particle size of the clay samples was determined by a modified Bouycous hydrometer method essentially as described in A.S.T.M. D4 2254T. Results from the particle size analyses were as follows:

Material minus-2-micron, e.s.d.

Weight, percent Unground clay 14.2 Ground clay 75.5

It is evident from these particle size data that the grinding process achieveda remarkable reduction of the particle size of the kaolinitic clay. Further evaluation of this ground clay for its suitability as a paper coating clay showed that the grinding process had improved the brightness of the clay and also had improved the gloss characteristics of the ground clay to an exceptionally high value. The remarkable amount of particle size reduction achieved in Example 1 was not anticipated and no indication of this intensive degree of grinding was indicated in earlier tests made in grinding equipment identical to that used in Example 1, but operated as an attrition scrubber without the use of a granular grinding media in the grinding charge. The results from such a test are given in Example 2.

Example 2 Weight, Weight,

grams percent Water 200 57. 1 Clay 7 150 42. 9

Total 350 100.0

About 0.5 gram of tetrasodium pyrophosphate was added to the grinding charge as a deflocculating reagent.

After the valve at the bottom of the container (FIG- URE l, valve 12) was closed, the water and deflocculating reagent were placed in the mill. The clay was added and the machine was run for 30 minutes, with the rotor turning at a rate sufiicient to give a peripheral speed of about 1400 feet per minute. After the 30-minute scrubbing period, the rotor was stopped; valve 12 was opened and the ground charge flushed out of the machine and collected in a container.

The particle size distribution was determined on a representative sample of the unground clay and also on a representative sample of the clay taken from the ground clay slurry. The method for determining the particle size of the samples was identical to that described in Example 1. Results from the particle size analyses were as follows:

Material minus-2-micron, e.s.d.

Weight, percent Unground clay l4.2

Ground clay 15.1

Example 3 A standard commercially available Waring Blendor with water capacity of about 500 cc. was used for this test. The agitating blades in this device rotate at about 10,000 rpm. Another portion of the same Georgia kaolinitic clay used in Example 1 was used in Example 3. Tetrasodiurn pyrophosphate was used in the grinding charge as a deflocculating reagent, but no granular grinding media was used in the grinding charge for this test. The grinding charge had the following composition:

Weight, 1 grams Weight, percent Water Clay Total About 0.17 gram of tetrasodium pyrophosphate was added to the grinding charge as a deflocculaiting reagent.

The Water, clay, and tetrasodium pyrophosphate were placed in the blender container and the machine was run for 30 minutes. After the running period the machine was stopped and the slurry of water, clay, and reagent was flushed out of the machine into a container.

The particle size distribution was determined on a representative sample of the unground clay and also on a representative sample of the clay taken from the ground clay slurry. The method for deterrnim'ng the particle size of the samples was identical to that described in Example 1. Results from the particle size analyses were as follows:

Material minus-2-micron, e.s.d.

weight, percent Ungnound clay 14.2

Ground clay 27.1

it is evident from these data given in Example 3 that this type of high speed agitating equipment was not par:

ticularly effective in reducing the particle size of the clay.

internal diameter was used to grind a second and different sample of Georgia kaolinitic clay on a batchwise basis. The grinding charge used had the following composition:

About 13.6 grams of tetrasodium pyrophosphate was added to the grinding charge to deflocculate the clay in the slurry. The Ottawa quartz sand was sized to contain only minus-l4/plus-28-mesh (Tyler standard screen scale) size particles. The grinding media occupied 72 percent of the total volume of clay and sand in the charge. 7

After the valve at the bottom of the container (FiG' URE l, valve 12) was closed, the wated and defiocculating reagent for the charge were placed in the mill. Then the clay was added and the machine run for five minutes to thoroughly wet the clay. The quartz sand grinding media was then added to the mill and the rotor 9 was turned by hand until the grinding media was wet by the slurry of clay and water. The grinder was then started and the rotor turned at a rate sufiicient to give a peripheral speed 'of about 1400 feet per minute. After a. 30-minute period of grinding the rotor was stopped; valve 12. was opened and the grinding charge discharged and washed out of the mill onto a vibrating screen.

The ground clay was removed from the quartz sand grinding media in a manner similar to that used in Example 1. Particle size analyses were made on a representative sample of the unground clay that was used as feed tor the grinding process and a representative sample of the clay ground in the process. The method for determining the particle size of the samples was identical to that described in Example 1. Results from the particle size analyses were as follows:

Material minus-Z-micron, e.s.d.

weight, percent Unground clay 23.1 Ground clay 93.2

These particle size analyses data show that the grinding process greatly decreased the particle size of the kaolinitic cl-ay. Additional, evaluation of this ground clay indicated that the brightness of the clay was also increased considerably.

Example Weight, Weight, grams percent 369 as Clay 22a Ottawa quartz sand 750 56.1 Total 1, 335 100. 0

About 0.75 gram of sodium hexametaphosphate was added to theg ri-nding charge as a defi'occulating reagent. The grinding media comprised '76 percent of the total volume occupied by the clay and sand in the charge.

The procedure for placing the sample in the grinder, grinding the sample, removing the sample from the grinder and determining particle size of the test products was carried out in an identical manner to that described in Example 1.

Results from the particle size analyses were as follows:

Material minus-2-micron. e.s.d.

weight, percent Unground clay .Q 14.2 Ground clay 72.5

Thus excellent grinding of the kaolinitic clay was obtained with the use of the minus-48-mesh/plus-100-mesh sized quartz sand gninding media and by using the sodium hexametaphosphate as deflccculating reagent.

Example 6 The results obtained in this test demonstrated the effectiveness of using a diiferent material as grinding media in the grinding process. The same grinding machine and sample of Georgia kaolinit-ic clay as used in Example 1 was used in Example 6-. The grinding media used was zircon sand, sized to contain only minus-48- mesh/plus-IOO-mesh (Tyler standard screen scale) size particles. Tetrasodium pyrophosphate was used as a deflcccu-lating reagent in the grinding charge. The grinding charge had the following composition:

Weight, Weight,

grams percent 236 8. I lay 243 8. 4 Zircon sand 2, 420 83. 5

Total 2, 899 100.0

About 1.22 grants of tetrasodium pyrophosphate was added to the grinding charge as a deflocculating reagent. The grinding media comprised 84 percent of the total volume occupied by the clay and grind-ing media in the charge.

The procedure for placing the sample in the grinder, grinding the sample, removing the sample from the grinder and determining particle size analysis of the test products was carried out in an identical manner to that described in Example 1.

Results from the particle size analyses were as follows:

Material minus-.l-micron, e.s.d.

weight, percent Unground clay 14.2 Ground clay 87.3

Very good particle size reduction was achieved in the test by using a grinding media composed of zircon sand, sized to contain only minus-48-mesh/plus-10il-mesh particles.

A wet grinding test was made on a kaolinitic clay by the method described in U. S. Patent 2,581,414. The results from this test (given in Example 7) showed some particle size reduction; however, the rate of particle size reduction was considerably inferior to that produced by the method of our invention.

Example 7 An apparatus similar to that described in FIGURE 1 of US. Patent 2,581,414 was assembled by mounting two metal discs (3 /2-inch diameter) parallel to each other on a shaft. One disc was fixed at the end of the shaft and the second disc was located on the shaft 1% inches away from the first disc. A small electric motor supplied the power for rotating the shaft and the attached discs in a metal container of about 4 inches internal diameter.

The grinding charge of clay, water, reagent, and granular grinding media was placed in the container and the shaft with discs was rota-ted for 39 minutes at 1200 r.p.m. At this rate of rotation the peripheral speed of the discs was about 1100 feet per minute.

A portion of the same Georgia k-aolinitic clay as used in Example 1 was also used in Example 7. The composition of the grinding charge was as follows:

About 0.5 gram of tetrasodium .pyrophosphate was added to the grinding charge to deilocculate the clay in the slurry. The Ottawa sand was sized to contain only minus-l4-mesh/plus-28-mesh (Tyler standard screen scale) size particles. The grinding media occupied about 76 percent of the total volume of clay and sand in the charge.

After the specified period of grinding was completed the ground clay was removed from the quartz sand grinding media in a manner similar to that used in Example 1. Particle size analyses were made on a representative sample of the unground clay that was used as feed for the grinding process and a representative sample of the clay ground in the test. The method for deter-mining the particle size of the samples was identical to that described -in Example 1. Results from the particle size analyses were as follows:

Material minus-Z-micron e.s.d.

weight, percent Unground clay 14.2

Ground clay 39.7

The results obtained in this example show that a moderate amount of grinding of the clay was achieved, but the reduction in particle size obtained from a 30-minute grinding period was considerably inferior to that obtained in a 30-minute grinding period through the use of our process carried out according to Example 1.

It should be noted that in this test the peripheral speed of the rotating mechanism was only 79 percent as fast as used in previous tests. However, in comparing the results obtained, the diference in the amount of material reduced to minus-Z-micron size (as in Example 4) was too great to be accounted for by the difference in the speed at which the mechanism in Example 7 was operated.

It is apparent from the foregoing description that the art of grinding materials to extremely fine size is advanced by this invention, and that the high degree of particle size reduction was not expected. The grinding process is carried out in simple, inexpensive equipment while using readily available and inexpensive grinding media, and is carried out with relatively low consumption of power. This improvement in the art of fine grinding is or" great importance and our invention is a valuable contribution to the technology of grinding.

Since many widely dififering embodiments of the invention will occur to one skilled in the art, such as the use of container vessels of different shapes, rotor and stator blades at various angles, various rotor peripheral speeds, and horizontal as well as vertical arrangement of grinding units, the invention is not limited to the specific details illustrated and described, and various changes may be made therein without departing from the spirit and scope thereof. Similarly, it is known that the method may by applied to many other materials, such as talc, pyrophyllite, mica, graphite, etc.

We claim:

1. A method for grinding solids to particle size less than 37 microns equivalent spherical diameter, comprising agitating by continuously moving about at a high speed for a predetermined period of time a slurry composed of the solids to be ground, a liquid, and a granular grinding medium harder than the said solids to be ground, and including particles having sizes within the entirety of the range from 705 to 75 times larger in diameter than the diameter desired for the upper limiting size of the particles in the final ground product, removing during agitation a portion of the slurry composed of ground solids and liquid, separating from said portion a product of ground solids of desired diameter in liquid, and returning the coarser portion of the said removed slurry along with additional solids to be ground and make-up liquid to the first mentioned slurry being agitated.

2. A method as described in claim 1, wherein the liquid in the grinding charge forms an aqueous slurry.

3. A method as described in claim 2 wherein a kaolinitic clay is the solid material to be ground.

4. A method as described in claim 3 wherein a deilocculating reagent is included in the grinding charge.

5. A method as described in claim 1 wherein a quartz sand is used as the grinding medium.

6. A method as described in claim 5 wherein the liquid in the grinding charge forms an aqueous slurry.

7. A method as described in claim 6 wherein the grinding medium will occupy from 48 to 95 percent of the volume of the solids in the grinding charge.

8. A method as described in claim 7 and a defiocculating reagent is added to the grinding charge.

9. A method as described in claim 8 wherein a kaolinitic clay is the solid material to be ground.

10. A method for grinding solids to particle size less than 37 microns equivalent spherical diameter, comprising agitating by continuously moving about at a high speed for a predetermined period of time a slurry composed of the solids to be ground, a liquid, and a granular medium harder than the said solids to be ground, and including particles having sizes within the entirety of the range from 705 to 75 times larger in diameter than the diameter desired for the upper limiting size of the particles in the final ground product.

11. A method as described in claim 10, wherein the liquid in the grinding charge forms an aqueous slurry, and a kaolinitic clay is the solid material to be ground therein.

12. A method as described in claim 10, wherein the liquid in the grinding charge forms an aqueous slurry, and the grinding medium will occupy from 48 to 95 percent of the volume of the solids in the grinding charge.

13. A method as described in claim 12, wherein a quartz sand is used as the grinding medium.

14. A method for grinding solids to particle size less than 37 microns equivalent spherical diameter comprising agitating by continuously moving about at a high speed for a predetermined period of time a slurry comprised of the solids to be ground, a liquid, and a deflocculating reagent, thereafter adding to said agitated slurry a granular medium harder than the said solids to be ground, and including particles having sizes within the entirety of the range from 705 to times larger in diameter than the diameter desired for the upper limiting size of the particles in the final ground product, and agitating the slurry comprising the grandular medium for a second and substantially longer predetermined time.

15. A method as described in claim 14, wherein the liquid produces an aqueous slurry in which a kaolinitic clay is the solid material to be ground.

References Cited in the file of this patent UNITED STATES PATENTS 1,956,293 Klein et a1 Apr. 24, 1934 2,520,320 Lyons et a1 Aug. 29, 1950 2,581,414 Hochberg Jan. 8, 1952 2,789,772 Williamson Apr. 23, 1957 2,855,156 Hochberg et al Oct. 7, 1958 2,956,753 Williamson Oct. 18, 1960 

14. A METHOD FOR GRINDING SOLIDS TO PARTICLE SIZE LESS THAN 37 MICRONS EQUIVALENT SPHERICAL DIAMETER COMPRISING AGITATING BY CONTINUOUSLY MOVING ABOUT AT A HIGH SPEED FOR A PREDETERMINED PERIOD OF TIME A SLURRY COMPRISED OF THE SOLIDS TO BE GROUND, A LIQUID, AND A DEFLOCCULATING REAGENT, THEREAFTER ADDING TO SAID AGITATED SLURRY A GRANULAR MEDIUM HARDER THAN THE SAID SOLIDS TO BE GROUND, AND INCLUDING PARTICLES HAVING SIZES WITHIN THE ENTIRETY OF THE RANGE FROM 705 TO 75 TIMES LARGER IN DIAMETER THAN THE DIAMETER DESIRED FOR THE UPPER LIMITING SIZE OF THE PARTICLES IN THE FINAL GROUND PRODUCT, AND AGITATING THE SLURRY COMPRISING THE GRANDULAR MEDIUM FOR A SECOND AND SUBSTANTIALLY LONGER PREDETERMINED TIME. 